This invention relates generally to surgical retractors and more particularly to a surgical retractor assembly having an embedded sensor for detecting metabolic activity and viability of the underlying tissue during surgery and a monitoring system for displaying a signal representative of this activity.
A surgical retractor is used to retract or spread apart overlying tissue and/or organs in order that an internal organ or an underlying organ or particular piece of tissue may be exposed during a surgical procedure. It basically acts as an extra hand. The retractor is in turn usually held in place by a mechanical arm arrangement fixed to the surgical table or to a portion of the patient's body. Surgical retractors typically take the form of a blade having a generally flat shape that is malleable so as to allow the surgeon to bend the blade into a particular shape that may be required in a particular case.
Some types of tissues or organs, such as the brain, are extremely sensitive to externally applied pressure and movement. Brain tissue is the most sensitive of any body tissue to movement and pressure. To protect the brain from damage, the brain is encased in a durable sheath generally known as the dura mater which is in turn enclosed within the skull. In many brain procedures a portion of the skull is removed and an incision made through the dura mater thus exposing the delicate brain tissue. The brain tissue is then carefully separated and retracted in order for the surgeon to view and reach the underlying tissue. A surgical retractor is used to hold the brain tissue in a retracted position so as to expose the underlying portion of the brain during the procedure. The pressure exerted by the retractor often results in impaired blood flow and oxygen starvation of the underlying brain tissue. This, in turn, leads to tissue injury. Accordingly, pressure exerted by the retractor must be minimized and carefully controlled.
Neurosurgeons have historically relied on feel, experience, and judgement to gauge the degree of retraction pressure which is safe and tolerable to the underlying brain tissue. However, the brain tissue in some patients is more sensitive than others and therefore what may be safe for one patient may not be for another. Furthermore, in an individual patient, alterations in blood oxygenation, cerebral blood flow and brain metabolic demands during surgery may make a degree of brain retraction (and hence compression) which was well tolerated at one point during surgery dangerous and harmful at another.
Several methods have been developed to try to measure the pressure being applied by a retractor utilized in these types of operations. One such retractor is disclosed in U.S. Pat. No. 4,263,900. This instrument includes an inflatable envelope between the blade and the tissue having a pair of electrodes positioned therein which intermittently contact each other in response to fluid pressure inside the enclosure. Contact closure increases fluid pressure within the enclosure. When the contacts open, the pressure increase is stopped. Therefore, when a balance is reached, the internal pressure corresponds to the pressure being applied by the retractor to the underlying tissue. Another approach is to place a piezo-resistive strain gauge on the retractor to directly sense and measure the force being applied.
However, pressure measurement is only an indirect gauge of the safety of a particular degree of brain retraction. It is the effect of that pressure on the blood flow, oxygenation and metabolism of the tissue underlying the retractor which is important. Hence, it is these parameters that should be monitored to prevent overretraction and resulting tissue damage.
It is therefore an object of the present invention to provide a surgical retractor having an integral sensor in the blade to detect and monitor the oxygen content of the tissue underlying the retractor blade.
It is another object of the present invention to provide a surgical retractor having a sensor embedded therein to directly monitor micro blood flow in the underlying tissue.
It is another object of the present invention to provide a retractor having a removable sensor in the blade which monitors the intracellular metabolic state of the underlying tissue.
It is another object of the present invention to provide a monitoring system for displaying measured parameters of the activity of the tissue underlying the surgical retractor.